The invention relates to an ink jet printer that records images and characters on a recording sheet by jetting ink droplets from nozzle openings by extension and contraction of piezoelectric vibration elements. More particularly, the invention is directed to an art for driving the piezoelectric vibration elements of an ink jet recording head.
There are two types in the jet printer. A piezoelectric vibration type is designed to jet ink droplets by expanding and contracting pressure chambers by piezoelectric vibration elements, and a thermal type is designed to arrange heating means within pressure chambers and jet an ink by taking advantage of pressure produced by instantly gasifying the ink while supplying electric energy to the heating means.
Particularly, the recording head of the former type that produces ink droplets by extension and contraction of the piezoelectric vibration elements repeats at a predetermined cycle the process of: contracting the piezoelectric vibration elements so as to expand the pressure chambers before jetting the ink, thereby introducing the ink into the pressure chambers; extending the piezoelectric vibration elements so as to correspond to a print signal inputted after a predetermined time elapses thereafter; and jetting ink droplets from nozzle openings by the pressure thereby produced.
Since dots are formed on a recording sheet by splashing ink droplets a predetermined distance between the head and the sheet, the print quality is affected by the ink droplet splashing speed with the ink droplets being affected by the gravitational force, the speed of movement of the recording head, and the like. To improve the print quality, the ink droplet splashing speed must be increased as much as possible.
Since the ink droplet splashing speed depends on the piezoelectric vibration element extension speed which is used for the recording head, the speed of varying a drive signal voltage must be increased as much as possible.
However, the piezoelectric vibration element is subjected to residual vibration at the self resonance frequency after an ink droplet has been jetted once the extension speed thereof exceeds a certain value. Thus, a so-called "satellite" with small ink droplets is produced after the ink droplet has been jetted, from which arises a problem of further impairment of the print quality.